The present invention relates generally to stratified two-stroke internal combustion engines and more particularly to rotary valves for timing of engine parameters.
In two-stroke engines, a fuel-air mixture is generally inducted into the crankcase. At selected times, the fuel-air mixture is then communicated from the crankcase to the combustion chamber via transfer passages. For improved engine efficiency, it is desirable to control the volume of fuel-air mixture provided to the crankcase. Typically, the flow of gasses into the crankcase has been controlled either by using the piston to open and close the mixture port to the engine, by having an external rotary valve placed adjacent to the crankcase that times the mixture input port, or using a counterweight to time the mixture input port. Another solution uses electronic fuel injectors to supply selected amounts of fuel to the transfer passages during a portion of the air intake stroke.
Stratified two-stroke engines use pure air trapped in the transfer passages to help scavenge combustion gasses from the engine before the new fuel-air mixture charge enters the combustion chamber. Generally, the fuel-air mixture and the scavenging air are inducted into the engine during the upstroke of the piston. The transfer passages receive pure air from an air port, while the crankcase receives a fuel-air mixture from a mixture port. Alternatively, if an electronic fuel injector is used, the fuel may be supplied to the transfer passages at selected times during a portion of the air intake. After fuel injection, additional pure air fills the transfer passage. In many stratified two-stroke engines, the scavenging air may overflow the transfer passages and dilute the fuel-air mixture in the crankcase, or the fuel-air mixture in the crankcase may begin to mix with the scavenging air in the transfer passages. It is desirable to avoid diluting the fuel-air mixture in the crankcase with pure air from the air port. Furthermore, to minimize hydrocarbon emissions, it is desirable to keep the air in the transfer passages as free from the fuel-air mixture as possible.
Each of the four methods of controlling the intake of fuel-air mixture discussed above presents disadvantages. Piston ported mixture ports can only be opened and closed symmetrically with respect to the piston's top dead center (TDC) position. This means that if a mixture port opens at a crank angle θ before piston TDC, it will close at an angle θ after piston TDC. In many instances, it may be desirable to open the mixture port at one angle before piston TDC and close the mixture port at a different angle after piston TDC. External rotary valves, on the other hand, require additional structural housing components affixed to the crankcase. Accordingly, rotary valves add to the size and weight of an engine, as well as adding additional components and complexity. Counterweights, also known as crank webs, used for timing control are typically made from cast metal. In order to properly time the input of gasses to the crankcase, the counterweights are altered in shape so that they are substantially cylindrical instead of the conventional off-center or mushroom-like shape. While counterweights designed for timing control may be used to prevent mixing of the scavenging air and the fuel-air mixture in the crankcase, the altered shape requires additional material, which leads to extra weight and cost. Additionally to properly control the timing of opening and closing of the fuel-air mixture port or transfer passage, the counterweights may require complicated cast passages or voids. Furthermore, the unbalanced shape of conventional counterweights is well suited to balancing out the forces created by the reciprocating piston and rotating crankshaft. However, the nearly cylindrical shape required for the altered counterweights makes the counterweight itself more balanced and thus less capable of offsetting or balancing the other engine forces. Thus, fabricating and balancing counterweights is difficult and costly. Engines using electronic fuel injection require complicated mechanisms and software to control the timing and duration of the injection. Thus, electronic fuel injection adds to both the cost and complexity of the engine.